Design And Implementation Of Current Field Through-the-earth Spread-spectrum Communication System For Mine

Recently,as a result of coal mine and gold mine safety accidents often happen,our country pay more and more attention to people’s rescue.Traditional wired communication has its own limitations.If an unexpected event occurs,wired devices under the mine will be destroyed,and it is difficult for rescuer communicating with trapped person,which made it difficult to locate and rescue trapped person.Thereby making the terrestrial wireless communication system particularly important.Because of the superiorities of high efficiency and invulnerability features,the communication based on electric field theory has become a through-earth communication method which presents good growth prospects.The main task in this paper is to design through-the-earth spread-spectrum communication system for mine based on current field theory.Firstly,the working principle of this system would be introduced,including current field and spread-spectrum communication theory.And then analysed and constructed the mathematical model of current field communication,and electric diloles were chosen as receiver and transmission antenna.Due to the special mine environmental factors and the analysis of the mine channel,the communication frequency was restricted to below the very low frequency band.Then,the design of the system could be divided into digital part and analog part.Analog part finished analog signal processing,including power amplifier,weak signal reception,signal filtering and so on.It would employ some simulation software such as Candence、Filterpro 、 LTspice when carry out the design of circuit in order to make sure the best performance.And then drew circuit diagram and printed circuit board with software Altium Designed.The digital part of this system used the idea of software radio.As the core of digital part,modulation、demodulation、spread spectrum、despreading and filtering process were all completed in FPGA,and configure peripheral modules such as AD / DA and voice codec.Modulation mode used BPSK which has advantages of high band utilization,low bit error rate,etc.This paper would introduce different ways to spread spectrum: direct sequencing and soft spread spectrum.In order to realise emergency rescue,you could send not only digital messeage but also voice message,as voice messeage will spend too much bandwidth resource and restricted by the limitation of mine channel,the system cannot afford enough bandwidthfor voice transmission.So it cannot send speech signal directly,the speech signal is subjected to compression encoding processing before transmitting them.As for digital messeage and speech signal,this paper will use different frame headers to distinguish them.Finally,the function debugging of the system and field testing would be conducted.It would debug every module in laboratory room firstly,in order to test the performance of each module,and then carried out field testing to make sure its stability and reliability.